1. Field of the Invention
The present invention relates to a circuit chip, and, more particularly, to a high frequency circuit chip used in a multi-chip module (MCM) which operates in the microwave and millimeter wave bands, and a method of producing such a high frequency circuit chip.
2. Description of the Related Art
An example of a conventional high frequency circuit chip (passive circuit chip) used in multi-chip modules (MCM) which operate in the microwave and millimeter wave bands, is shown in FIG. 6.
The high frequency circuit chip includes lumped constant elements such as a wiring pattern 52 (such as a distributed constant line or a microstrip line), a meander inductor 53, an interdigital capacitor 54, and a thin film resistor 55 disposed on the upper surface (the front surface) of a substrate 51, a back-surface electrode (conductor layer) 56 provided substantially on the entire lower surface (back surface) of the substrate 51, and a through hole 57 which connects the wiring pattern 52 provided on the front surface of the substrate 51 and the back-surface electrode (conductor layer) 56 to each other.
In the high frequency circuit chips of the related art, conductor patterns used as the wiring pattern 52, the meander inductor 53, and the interdigital capacitor 54, for example, are commonly formed by printing electrically conductive paste such as Ag paste and Cu paste and firing, or by wet-etching Au plating films having a thickness of 2 μm to 5 μm). The minimum line width of wiring patterns formed by printing is 100 μm, and that obtained by etching is 20 μm.
As the substrate 51 in the high frequency circuit chip shown in FIG. 6, a Teflon substrate (εr: 2 to 3) and an alumina substrate (εr: about 10) are commonly used. To reduce the size of the high frequency circuit chip, a ceramic substrate made of barium titanate or other suitable material as a major raw material, having a high dielectric constant, and to operate in the microwave band is effectively used. However, to produce the most accurate wiring pattern, the dimensional precision and the shape accuracy, the aforementioned Teflon or alumina substrate is used.
Moreover, in the high frequency circuit chip of the related art, a perforating through-hole 57a is provided in some cases, and a filled through-hole 57b is provided in other cases. The perforating through-hole 57a is provided by perforating the substrate by laser beam processing or other suitable method, and then, providing an electrode film 59 on the inner wall of the perforation 58 by sputtering or vacuum deposition as shown in FIG. 7, whereby the front and back surfaces of the substrate are electrically connected to each other, as shown in FIG. 7. The filled through-hole 57b in which a filling electrode 60 is embedded into the perforation 58 is provided by plating or filling electrically conductive paste as shown in FIG. 8.
In the aforementioned high frequency circuit chip, the wiring pattern is commonly formed by printing or etching. As described above, the minimum line width of wiring patterns is 100 μm when printing is used, and is 20 μm when etching is used. Recently, the size of electronic devices has decreased. Thus, wiring patterns having reduced line widths are required.
To provide wiring patterns having reduced line widths, reactive ion etching (RIE), ion milling methods, and other suitable methods are used. However, the apparatuses required for reactive ion etching and ion milling are expensive and, moreover, the substrates are chemically damaged, which deteriorates the electrical characteristics of such chips.
In high frequency circuit chips having a perforating through-hole 57a type (FIG. 7), a resist cannot be uniformly coated, which makes it difficult to form a fine wiring pattern.
Moreover, in high frequency circuit chips having a filled through-hole 57b (FIG. 8) the electrode 60 filled into the through-hole 57b is convex or concave with respect to the main surfaces of the substrate 51, which makes it difficult to uniformly coat a resist and form a fine wiring pattern.
Au of which the resistance is low is generally used for the wiring pattern 52, the electrode film 59 or the filled electrode 60 of the through-hole. However, Au is expensive which substantially increases the cost of the product.